Among the conventionally-known vehicle front body structures are ones which include, on each of left and right sides of the vehicle body, an upper member disposed laterally outwardly of a corresponding (i.e., left or right) front side frame, inner and outer impact absorbing sections provided on respective front end portions of the front side frame and upper member in spaced-apart relation to each other in a width direction of the vehicle body, and a bumper beam fixed to the inner and outer impact absorbing sections.
One example of such vehicle front body structures is disclosed in Japanese Patent Application Laid-Open Publication No. 2007-190964 (JP 2007-190964 A), which will be described below with reference to FIG. 20 hereof. FIG. 20 is a view illustrating how impact energy caused by an offset collision (i.e., collision impact energy) is absorbed in the conventional vehicle front body structure, only with respect to one of the left and right sides (left side in the illustrated example of FIG. 20) of the vehicle front body structure. In the vehicle front body structure 200, inner and outer impact absorbing sections 203 and 204, constituting an impact absorbing unit of the structure 200, are provided on respective front end portions of the left front side frame 201 and upper member 202; thus, the impact absorbing unit can have an increased overall width. Consequently, in case an impact load is applied to a bumper beam 205 as indicated by white arrow f, each of the impact absorbing sections 203 and 204 can be prevented from undesirably tilting sideways, so that the applied impact load f can be transmitted via the absorbing sections 203 and 204 to the left front side frame 201 and upper member 202.
Inner side wall portion 204a of the outer impact absorbing section 204 is spaced outwardly from an outer side wall portion 201a of the left front side frame 201 by a distance S. Thus, when another vehicle 210 has collided against the vehicle in question (i.e., vehicle where the vehicle front body structure 200 is employed) with an offset (e.g., leftward offset) from a longitudinal centerline extending centrally through the width of the vehicle in question, a collision impact load applied to the bumper beam 205 is transmitted to the left front side frame 201 and left upper member 202 by way of the absorbing sections 203 and 204 as indicated by arrow a and b.
Such an offset collision would deform the bumper beam 205 as depicted by imaginary lines, and thus, the collision impact load would act toward the longitudinal centerline of the vehicle body as indicated by arrow c. This impact load is applied to the left front side frame 201 and would bend the left front side frame 201 in a direction indicated by arrow d. For this reason, a stiffener (partitioning plate) functioning as an reinforcing member is provided on the left front side frame 201 for preventing the side frame 201 from being bendingly deformed as indicated by arrow d. However, if the left front side frame 201 is reinforced with such a stiffener, the left front side frame 201 would undesirably increase in weight.
Also known in the art are vehicle front body structures where, on each of the left and right sides of the front vehicle body, respective front end portions of the front side frame and upper member are interconnected via a connecting frame and an acceleration sensor is provided on the connecting frame. Occurrence (i.e., presence/absence) of a collision is determined on the basis of acceleration (deceleration) detected by the acceleration sensor. Once it is determined that a collision has occurred to the vehicle, an airbag is developed or deployed to protect a vehicle occupant. One example of such vehicle front body structures is disclosed in Japanese Patent No. 3930004 (JP 3,930,004 B).
Also known in the art are vehicle front body structures which can absorb impact energy caused not only by a low-speed collision but also by a high-speed collision. Thus absorbing impact energy caused by a low-speed collision can protect an object having collided against the vehicle in question, while absorbing impact energy caused by a high-speed collision can protect a vehicle occupant. By providing such vehicle front body structure with the acceleration sensors as disclosed in JP 3,930,004 B, an airbag can be deployed into a vehicle compartment once acceleration detected by any of the acceleration sensors exceeds a predetermined airbag-deploying acceleration threshold value; this arrangement can protect a vehicle occupant even more effectively.
In order keep the airbag undeployed at the time of a low-speed collision and to get deployed only at the time of a high-speed collision, it is necessary to set an appropriate airbag-deploying acceleration threshold value with a high accuracy. The acceleration produced by impact energy would fluctuate to some degree. Therefore, in order to set the airbag-deploying acceleration threshold value with a high accuracy, it is preferable to set the airbag-deploying acceleration threshold value within a range in which the acceleration presents a sharp increase, i.e., within a range of a great (and hence clear) acceleration change rate. However, because the vehicle front body structures have a low-speed-collision absorbing function capable of absorbing impact energy caused by a low-speed collision, the low-speed-collision absorbing function would undesirably work at the time of a high-speed collision. Therefore, it is difficult to secure a great acceleration rate for a high-speed collision and set the airbag-deploying acceleration threshold value within the range of the clear acceleration change rate. Therefore, it has been difficult to accurately determine, on the basis of a prestored (preset) airbag-deploying acceleration threshold value, proper timing (or acceleration level) at which the airbag is to be deployed